In a power train control system, a vehicle control system, a body control system, and an information communication system mounted in a vehicle, a lot of sensors are used to sense an ambient environment of the vehicle and the operating states of various facilities, such as, the temperature, the burning state in an engine, the operating states of various actuators, an air intake/exhaust state, the posture of the vehicle, and the state of a battery. Various sensors convert a change in a specific physical or chemical quantity into a change in an amount of electricity, that is, a change in a voltage, a current, or an electrostatic capacitance on the basis of various physical or electrochemical principles.
Generally, a voltage output level sent from a sensor is often so small as to range from several millivolts to several hundreds of millivolts. On the other hand, an input dynamic range or a convertible voltage range of an analog-to-digital (A/D) converter incorporated in an automobile microcomputer or the like is, generally, a range from 0 V to 5 V. If a feeble signal sent from the sensor is A/D-converted as it is, the dynamic range of the A/D converter cannot be effectively utilized. Therefore, an amplifier that exhibits an appropriate magnification is often disposed in a stage preceding the A/D converter.
Moreover, a variance of one sensor from the others caused during manufacture brings about a variance in the sensitivity of the sensor or an offset. Therefore, a correction or compensation means is generally included in a processing circuit. A typical correction or compensation means is realized with a microcomputer in an electronic control unit (ECU). After each sensor is connected to the ECU, a signal sent from the sensor is amplified at an appropriate fixed magnification in the ECU, and A/D-converted. The ECU then executes a correctional or compensational arithmetic operation for the resultant signal.
When the foregoing means compensates an offset voltage derived from a sensor, simple addition/subtraction processing should merely be performed. However, for correcting sensitivity, multiplication/division processing is needed. If the number of sensors is large, a processing load for the correctional or compensational arithmetic processing increases.
In recent years, a processing load incurred by an ECU has greatly increased along with the complexity in vehicle control. A movement of separating from the ECU by incorporating a preprocessing circuit for a sensor signal in a sensor has been accelerated. Moreover, the number of harnesses over which sensors and ECUs are interconnected is increasing along with an increase in the number of sensors. The number of cases where the individual sensors and ECUs are interconnected over an onboard LAN (CAN or LIN) for which a certain communications protocol is defined, but are not interconnected over independent harnesses is increasing.
In such a configuration, each of the sensors executes not only amplification of a sensor output signal and A/D conversion but also simple processing succeeding the conversion. The results of the execution, that is, a digital value is sent to each ECU over a bus of the LAN. In this case, if the sensors simultaneously execute sensor variance compensation and send their sensor output values, which have been compensated and normalized, to the respective ECUs, it would be more preferable.
Broadly, a means for performing correction or compensation in a sensor falls into two types. One of the types is a type of means for correcting or compensating an analog signal that has not been A/D-converted, and the other type is a type of means for executing correctional or compensational arithmetic processing for an A/D-converted value or a digital value similarly to correction or compensation to be performed in an ECU.
Talking of correction processing and compensation processing, compensation of an offset that is a stationary deviation in a sensor output is addition/subtraction processing, and can be implemented using a relatively simple means. Supposing the addition/subtraction processing is analog processing, it can be implemented using a D/A converter and a subtraction processing circuit which is easily constructed using an operational amplifier. Supposing the addition/subtraction processing is digital processing, it can be implemented using an adder alone. In contrast, sensitivity correction requires multiplication/division processing. Supposing the multiplication/division processing is analog processing, a variable gain amplifier or a voltage division circuit that supports a variable voltage division ratio is needed. Supposing the multiplication/division processing is digital processing, a multiplier or a micro-processing unit (MPU) is needed. In either case, compared with the offset compensation means, the sensitivity correction means will become large in a circuit scale.
JP-A-2003-87068 and JP-A-2003-218650 (corresponding to U.S. Pat. No. 6,714,075) have introduced variable gain amplifiers. The variable gain amplifier described in JP-A-2003-87068 employs an operational amplifier and an R-2R resistor circuit and supports a variable gain according to an N-bit digital signal. Moreover, the variable gain amplifier described in JP-A-2003-218650 employs a multi-bit resistor string.
In both of the variable gain amplifiers, a variation width of a gain or an adjustable width thereof is reduced by increasing the number of resistors. Therefore, although a gain can be designated in finer steps, an increase in an occupied area in a layout is unavoidable. Moreover, the precision in a gain depends on the specific precision in a resistance. However, when an attempt is made to improve the specific precision in a resistance in an LSI, the size of a resistor has to be increased. This leads to a further increase in the occupied area in a layout.
Thus, it is require to provide a variable gain amplifier that can attain a gain permitting an arbitrary resolution although the variable gain amplifier is realized with one circuit of a small circuit scale without the employment of a resistor string, and a D/A converter.